Free-cutting aluminum alloys were developed from standard heat treatable alloys, to which additional elements for forming softer phases in the matrix were added. These phases improve the machinability of the material during cutting by obtaining a smooth surface, while requiring decreased cutting forces and providing decreased tool wear. Chip breakage is also especially improved.
These softer phases are formed by alloying elements that are not soluble in aluminum, do not form intermetallic compounds with aluminum and have low melting points. Elements with these properties are lead, bismuth, tin, cadmium, indium and some others, which are not applicable for practical reasons. Said elements added individually or in combinations are precipitated during solidification in the form of globulite inclusions having a particle size from a few .mu.m's to some tens of .mu.m's.
The most important free-cutting aluminum alloys are:
Al--Cu with 0.2-0.6 wt. % Pb and 0.2-0.6 wt. % Bi (AA2011); PA1 Al--Cu--Mg with 0.8-1.5 wt. % Pb and up to 0.2 wt. % Bi (AA2030); and PA1 Al--Mg--Si with 0.4-0.7 wt. % Pb and 0.4-0.7 wt. % Bi (AA6262). PA1 a) as alloy elements: PA1 b) as impurities: PA1 c) the balance substantially 100% aluminum. PA1 1.sup.st group: 0.40 wt. % Sn to 0.70 wt. % Sn PA1 2.sup.nd group: 0.71 wt. % Sn to 1.00 wt. % Sn PA1 3.sup.rd group: 1.01 wt. % Sn to 1.30 wt. % Sn PA1 4.sup.th group: 1.31 wt. % Sn to 1.60 wt. % Sn PA1 5.sup.th group: 1.61 wt. % Sn to 1.90 wt. % Sn
In these alloys, inclusions are formed for the purpose of easier machinability, especially through the use of lead and bismuth. Recently, there has been a tendency to replace lead with other elements because of risks to human health and for ecological reasons. As substitutes, tin and partly indium are most frequently used. The possibility of using tin in aluminum free-cutting alloys has been well-known for a long time. Tin was one of the first elements to be added to aluminum free-cutting alloys in amounts up to 2 wt. %. In practice, the use thereof, on a larger scale, has never taken place because of an alleged impairment of corrosion properties, poorer alloy ductility and high price. Recently, tin has been added, especially to alloys of the groups Al--Mg--Si (AA6xxx series) and Al--Cu (AA2xxx series) containing--when in standard form--lead and bismuth, or lead only.
Alloys with tin should have similar or better properties as to microstructure, workability, mechanical properties, corrosion resistance and machinability in comparison with standard alloys. The formation of suitable chips of alloys with tin depends--similarly as in alloys with lead and bismuth--on the effect of inclusions for easier cutting upon the mechanism of breaking the material during cutting.
Earlier investigations and explanations of the mechanism of breaking chips have been based particularly on alloys containing lead and bismuth. Both elements form softer phases in a harder basis and retain their chemical and metallographic characteristics. At discontinuity sites, cohesion forces are weaker and, thus, the desirable breaking of chips during machining is facilitated. The distribution of globulite phases should be fine and uniform. A simultaneous addition of smaller amounts of two or more elements insoluble in aluminum has a greater effect upon machinability than the addition of one element. The elements are present in globulite phases in ratios equaling the analytical averages thereof.
It is known on the basis of practical experience that the breaking of chips is best at an eutectic composition of the elements insoluble in aluminum. Thus, the opinion prevails that a suitable breaking of chips is a result of the melting of said inclusions at temperatures attained during the working of the material by turning, boring, etc.